A light emitting diode is a device that generates and emits light by having electrons excited to a conduction band and holes excited to a valence band recombine. For emitting light, a light emitting diode has a specific multiple quantum well structure. A multiple quantum well structure has multiple thin layers of alternating well and barrier layers. That is, electrons and holes are confined in the well layers formed between the barrier layers, and light is emitted from recombination thereof. This is an effect of quantum confinement.
Accordingly, light generated in a light emitting diode is characteristically determined by bandgap energy of well layers. That is, when bandgap energy of well layers is large, wavelength of light generated therein is short, and when band gap energy of well layers is small, wavelength of light generated therein is long. Electrons confined in well layers are supplied by an n-type semiconductor layer, and holes are supplied by a p-type semiconductor layer. Accordingly, a light generating layer in a multiple quantum well structure form is sandwiched between an n-type semiconductor layer and a p-type semiconductor layer, forming junctions.
Semiconductor layers forming a light emitting diode are made of compound semiconductors. The semiconductor layers require higher bandgap energy than that of well layers in the light generating layer. This is to prevent light generated in the light generating layer from being absorbed in the semiconductor layer. Also, forming a p-type semiconductor layer becomes more and more problematic with decreasing wavelength of generated light. That is, even when a p-type layer containing a dopant is formed via a MOCVD process, etc, with an insufficient concentration of holes, efficiency is lowered. This is especially problematic for a light emitting diode that generates ultraviolet light.
For example, while AlN, AlGaN, or AlGaIn is used as a material for forming a p-type semiconductor layer, dopant activation in a p-type semiconductor layer is not easy, and efficiency is lowered due to low conductivity of the p-type semiconductor layer. Also, light absorption by a semiconductor layer based on p-type GaN is an important reason for decreased efficiency.
However, despite the aforementioned technically difficult problems, market demand for ultraviolet light emitting diodes is steadily increasing. Ultraviolet light emitting diodes have an advantage in terms of environmental friendliness compared to mercury lamps and may be used for elimination of pollutants in a human-living environment through water purification, air purification, disinfection, etc.
However, due to the technically challenging problems of the ultraviolet light emitting diode, efficiency is not sufficiently high, and, consequently, ultraviolet light emitting diodes are not widely applied for the market demand.